If you just strap your phone to an actual microscope (see my other post), you're basically just limited to the magnification that the microscopes achieve. And very good performing microscopes are very affordable by hacker news standards these days.

Stereo microscope magnifications are typical at about 20x to 40x, but I'll commonly go to 640x on the biological one, though in practice you bug out at the physical limits of optical microscopy just above 1000x.

His specific setup is presumably limited by the magnification provided by whatever his objective lens achieves.